The present invention is directed to the use of a high molecular weight (MW), crosslinked, water-soluble cationic polymer for controlling and preventing deposition of pitch and stickies in papermaking.
Cationic polymers have been used extensively in paper making as flocculants for improving retention and drainage and as coagulants or fixatives to control anionic trash and deposition of pitch and stickies. Among the most important and extensively used cationic polymers for deposit control are the quaternary ammonium polymers of diallyldialkyl ammonium compounds. It has been shown that the higher the molecular weight (MW) of the resulting cationic polymer, the more effective the polymer is as a flocculating agent. Normally a linear polymer of diallydimethyl ammonium choloride (DADMAC) is prepared. Polymerization using an azo initiator and/or with added inorganic salts (U.S. Pat. No. 5,248,744, U.S. Pat. No. 5,422,408, U.S. Pat. No. 4,439,580) has been used to achieve high MW. Use of crosslinking or branching agents in polymerization is another way to produce high MW cationic polymers. Polymerization with crosslinking agents can give high MW as well as structured polymers. A highly branched polyDADMAC can have better efficacy than a linear one of similar MW in certain types of applications.
U.S. Pat. No. 3,544,318 teaches that branched polyDADMAC is more effective than a linear polyDADMAC for electroconductive paper because the branched polymer imparts superior barrier properties to the electroconductive paper substrate, preventing solvent from diffusing into the paper.
Copending U.S. application Ser. No. 10/639,105 discloses crosslinked polyDADMAC by a Post-polymerization crosslinking reaction using water soluble radical initiators.
U.S. Pat. No. 3,968,037 shows that cationic polymers obtained by inverse (water-in-oil) emulsion polymerization with crosslinking and branching agents have surprisingly high effectiveness as flocculants and for the treatment of activated sewage sludge. The inventors used polyolefinic unsaturated compounds, such as tri and tetra-allyl ammonium salts, methylenebisacrylamide, as the crosslinking agents. They found that only ineffective products were obtained from solution polymerization containing a crosslinking agent.
European Pat. No. 0264710B1 claims that highly branched water-soluble polyDADMAC made from solution polymerization works better as a flocculant or defoaming agent for breaking oil-in-water emulsions. The patent teaches the art of making highly branched polyDADMAC. These branched polyDADMAC are made by adding 0.1 to 3.0 mole % of crosslinking comonomer such as methyltriallyl ammonium chloride (MTAAC) or triallylamine hydrochloride (TAAHCl) during progressive polymerization of DADMAC after monomer conversion has achieved at least 25% to 90%. A completely gelled product is obtained when the MTAA is added all at once in the beginning.
U.S. Pat. No. 4,100,079 discloses the use of copolymers of DADAMC and N-methylolacrylamide capable of post crosslinking as acid thickening agents in oil well drilling and fracturing for stimulating well production.
U.S. Pat. No. 4,225,445 discloses that branched DADMAC polymers are useful as acid thickeners in oil well drilling and fracturing operations. The branched DADMAC polymers are prepared by inverse emulsion polymerization of DADMAC with a crosslinker monomer such as triallylmethylammonium chloride.
U.S. Pat. No. 5,653,886 discloses the use of crosslinked DADMAC polymers as coagulants in suspensions of inorganic solids for mineral refuse slurry. The preferred high molecular weight crosslinked polyDADMAC for the application is prepared by copolymerization of DADMAC with acrylamide and triallylamine.
In studying interaction of cationic polyelectrolytes with counter anions, Ghimici et al (Journal of Polymer Science: Part B, Vol. 35, page 2571, 1997) found that the cationic polyelectrolyte sample with more branching or crosslinking had stronger binding with anionic counter ions. It is alleged that branching of the polycations creates regions with higher numbers of charged groups even at high dilution and consequently an increased number of counter ions is associated to them.
U.S. Pat. No. 5,989,382 uses a multifunctional (triallylamine) to make high molecular weight cross-linked poly-DADMAC, which can be used for pitch control in papermaking.
Pitch and stickies are interfering substances in the wet end of papermaking which can affect both the machine runnability and paper quality. The term “pitch” used here refers to colloidal dispersion of wood-derived hydrophobic particles released from the fibers during pulping process and is also called wood pitch. Wood pitch includes fatty acids, resin acids, their insoluble salts, and esters of fatty acids with glycerol, sterols, and other fats and waxes. Pitch deposit problems are seasonal because pitch composition varies by season and type of wood. The hydrophobic components of pitch, particularly triglycerides, are considered the major factors determining whether the presence of such pitch will lead to deposit problem. Deposit-forming pitch always contains a significantly high amount of triglycerides. The term “stickies” used here refers to sticky materials and interfering substances which arise from components of recycled fibers, such as adhesives and coatings. Stickies can come from coated broke, recycled waste paper for board making and de-inked pulp (DIP). The stickies from coated broke are sometimes called white pitch. Deposition of pitch and stickies often lead to defects in finished product and paper machine downtime causing lost profit for the mill. These problems become more significant when paper mills “close up” their process water systems for conservation and environmental reasons. Unless the pitch and stickies are continuously removed from the system in a controlled manner, these interfering substances will accumulate and eventually lead to deposit and runnability problems.
Seasonal pitch and stickies from recycled coated papers and de-inked waste paper cause major runnability problems resulting in lost production and hence lost profit for the mill. Pitch from wood is seasonal. Stickies from coated broke, recycled waste paper for board making and de-inked fiber occurs when these furnishes are being used. Technology in place today is based on fixation of the pitch or stickies to the fiber before they have a chance to agglomerate, or to coat the pitch or stickies with a polymer that makes them non-tacky and therefore unable to agglomerate.
Three chemical methods are commonly used by paper mills to control pitch and stickies deposit:                1) detackification        2) stabilization        3) fixation        
These methods are, however, not commonly used together since they may conflict with each other.
In detackification, a chemical is used to build a boundary layer of water around the pitch and stickies to decrease depositability. Detackification can be achieved by addition of pitch adsorbents such as talc and bentonite. However, pitch adsorbents such talc can end up contributing to pitch depositability if talc/pitch particles are not retained in the paper sheet surfactants, and water-soluble polymers.
In stabilization, surfactants and dispersants are used to chemically enhance colloidal stability and allow pitch and stickies to pass through the process without agglomerating or depositing. Cationic polymers are normally used as fixatives to control pitch and stickies through fixation. Nonionic polymers such as polyvinyl alcohol and copolymers such as polyacrylamide-vinyl acetate ( PCT Application WO 0188264) have been developed and used for stickies control through detackification. Hydrophobically modified anionic polymers such as a copolymer of styrene and maleic anhydride (U.S. Pat. No. 6,051,160) have been used for pitch deposit control through, most likely, the pitch stabilization mechanism.
In fixation, polymers are used to fix pitch and stickies to the fiber and remove them from the white water system. The interfering substances in papermaking systems are usually anionic in nature and are sometimes referred to as anionic trash or cationic demand. Anionic trash consists of colloidal (pitch and stickies) and dissolved materials that adversely affect the paper making in a variety of ways through deposit formation or interference with chemical additives. Removal of anionic trash by reducing cationic demand with a cationic polymer is a way of deposit control through fixation. The advantage of using cationic polymeric coagulants for pitch and stickies control is that the pitch and stickies are removed from the system in the form of microscopic particles dispersed among the fibers in the finished paper product.
U.S. Pat. No. 5,256,252 discloses a method for controlling pitch deposit using enzyme (lipase) with DADMAC polymers. A Filtrate turbidity test is used to evaluate performance for pitch control.
European Application No. 464993 discloses use of an amphoteric copolymer of DADMAC and acrylic acid salts for controlling natural pitch deposition. The polymers are not claimed for deposit control of stickies in recycle pulps and white pitch in coated broke. A filtrate turbidity test is one of the test methods used to evaluate the performance for pitch deposit control.
PCT Application No. WO 00034581 teaches that amphoteric terpolymers of DADMAC, acrylamide and acrylic acid can be used for treating coated broke to control white pitch. A filtrate turbidity test is used to determine performance of the polymers for white pitch deposit control.
European Application No. 058622 teaches a method for reducing or preventing the deposition of wood pitch during the papermaking process with an emulsion copolymer of DADMAC, DADEAC, acrylamide and acrylic acid. The DADMAC polymers used are not crosslinked.
U.S. Pat. No. 5,131,982 teaches use of DADMAC homopolymers and copolymers for coated broke treatment to control white pitch. The DADMAC polymers used are not crosslinked. The patent shows that crosslinked polyepiamines have better performance than a linear polyamine to give more turbidity reduction.
U.S. Pat. No. 5,837,100 teaches the use of blends of dispersion polymers and coagulants for coated broke treatment. Turbidity reduction testing is used to determine activity efficiency of the polymers.
U.S. Pat. No. 5,989,392 teaches the use of crosslinked DADMAC polymers for controlling anionic trash and pitch deposition in pulp containing broke. Pulp filtrate turbidity test is used to evaluate polymer performance in pitch deposition control. Improved efficiencies of solution crosslinked or branched polyDADMACs over conventional linear polyDADMAC are demonstrated. The crosslinked or branched polyDADMACs used are prepared using a polyolefinic crosslinking monomer such as triallylamine hydrochloride and methylene bisacrylamide.
European Application No. 600592 discloses a method to make low molecular weight crosslinked polyacrylate by post treatment with a radical initiator. The starting acrylate polymer solution is heated to a reaction temperature of 90° C. The desired amount of radical initiator is then added over a relatively short time period (15 to 30 minutes). The reaction temperature is maintained for an additional time, usually less than 2 hours, to use up the initiator added for crosslinking. The extent of crosslinking and MW increase is mainly controlled by reaction temperature, pH, the amount of initiator added, and the reaction time after the addition of the initiator. Initiator feed time is not used to control extent of crosslinking. The patent is related to making low MW crosslinked polyacrylates for dertergent and cleaning applications.
Crosslinking between the strong electrolyte polymeric radicals can be limited due to electrostatic repulsion. Ma and Zhu (Colloid Polym. Sci, 277:115-122 (1999) have demonstrated that polyDADMAC cannot undergo radical crosslinking by irradiation because the cationic charges repel each other. On the other hand, nonionic polyacrylamide can be readily crosslinked by irradiation. The difficulty of crosslinking polyDADMAC by organic peroxides was reported by Gu et al (Journal of Applied Polymer Science, Volume 74, page 1412, (1999). Treating polyDADAMC with a dialkylperoxide in melt (140 to 180° C.) only led to degradation of the polymer as being evident by a decrease in intrinsic viscosity.